The present invention relates generally to energy harvesting, and more particularly to circuits and methods for improving the efficiency of transferring energy from energy harvesting devices to energy storage devices and/or load devices.
Recently, various very low power integrated circuits that require extremely low amounts of operating current (often referred to as “nano-power” integrated circuits) have been developed which can be powered by very small amounts of power scavenged or harvested from ambient solar, vibrational, thermal, and/or biological energy sources by means of micro-energy harvesting devices. The harvested power then usually is stored in batteries or supercapacitors. (The term “nano-power” as used herein is intended to encompass circuits and/or circuit components which draw DC current of less than roughly 1 microampere.)
FIG. 1 shows an energy harvesting system 1-1 in which the output of a thermopile energy harvester 2-1 is coupled by a DC-DC converter (i.e., battery charger) 10 to a battery or supercapacitor 6, hereinafter referred to simply as battery 6. Thermopile energy harvester 2-1 can be modeled as a voltage source VT coupled between ground and one terminal 3A of a resistor Ri. Resistor Ri represents the internal thermopile resistance. An internal capacitance CT is coupled between conductor 3A and ground. (The internal resistance of typical commercially available thermopile harvesters may be from about 300 ohms to 1 kilohm.) The output of thermopile energy harvester 2-1 applies an output voltage Vout via conductor 3 to the input of DC-DC-converter 10, thereby supplying an output current Iout into DC-DC converter 10, which charges battery 6.
It can be readily shown that the transfer of power from thermopile energy harvester 2-1 is optimized if the thermopile harvester output resistance is equal to the equivalent input resistance Vin/Iin of DC-DC converter 10. The input impedance Vin/Iin of a typical boost, buck, or buck/boost converter or battery charger 10 typically does not have a fixed value, because DC-DC converter 10 operates so as to draw as much input current from its input as battery 6 can accept.
FIG. 2 shows another energy harvesting system 1-2 in which the output of an induction harvester 2-2 is coupled by DC-DC converter 10 to battery 6. Induction harvester 2-2 can be modeled as an EMF (electromotive force) voltage source εL coupled in series with inductances LFB and Lcoil and coil resistance Rcoil. (The internal resistance Rcoil of typical commercially available induction harvesters may be from about 1 ohm to 10 kilohms.) For optimum power transfer it is necessary to match the input impedance of DC-DC converter 10 to the output impedance of induction harvester 2-2.
FIG. 3 shows yet another energy harvesting system 1-3 in which the output of photo-voltaic solar cell 2-3 is coupled by a DC-DC converter 10 to battery 6. Solar cell harvester 2-3 can be modeled as a current source IPH coupled between conductor 3A and ground in parallel with diode D and leakage resistance RP. A series resistance RS (which typically may be from about 0.1 to 100 ohms) is connected between conductor 3A and harvester output conductor 3. For optimum power transfer it is necessary to match the input impedance of the DC-DC converter to the equivalent output impedance of solar cell harvester 2-3.
The Iout/Pout versus Vout curve in FIG. 3 indicates that if Vout is held above the turn-on threshold voltage of diode D, then all of the current Iout generated by solar harvester 2-3 flows through diode D to ground. Therefore, if the value of Vout is too high, the current generated by solar harvester 2-3 is wasted. Initially, Pout increases linearly with respect to Vout because the generated current Iout by the current source IPH in FIG. 3 is constant. As the generated current starts to flow through diode D, a maximum or peak value occurs in the Pout curve. Thus, if the value of Vout is too low, the full amount of available harvested power (Vout×Iout) is not being made available at the output of solar cell harvester 2-3, and if the value of Vout is high enough that some or all of the generated current IPH is flowing through diode D to ground, then the corresponding power is wasted and cannot be converted into a suitable output voltage and output current for charging battery 6.
The amount of power available from the harvesters of FIGS. 1-3 usually is small and unpredictable, so the intermediate energy storage (e.g., lithium batteries or supercapacitors) is often required in these applications to provide for system power needs when energy from the harvester is unavailable or insufficient. It is important that the small amounts of power available from nano-power harvesting devices be “managed” so that the harvested energy is transferred as efficiently as possible, with minimum power loss, to charge batteries or energize utilization devices.
Well known impedance matching techniques to optimize transfer of power from the output of a first circuit to the input of a second circuit involve matching the output impedance of the first circuit to the input impedance of the second circuit.
A common technique for optimizing the efficiency of harvesting power from large P-V (photo-voltaic) solar cells (which generate large amounts of power) is managed in order to charge batteries and/or energize utilization devices is referred to as “maximum power point tracking” (MPPT). Such MPPT optimization utilizes a digital processor to control the amount of power being harvested by executing various known complex MPPT algorithms to adjust the voltage values and current values of the DC-DC converter 10 so as to derive a “maximum power point”. Normally, MPPT tracking is performed by using a digital algorithm to adjust the input current Iin delivered from the harvester to the input of the DC-DC converter and determining or measuring the amount of power delivered to the input of the DC-DC converter.
For example, the MPPT algorithm might decrease the amount of current Iin, which might cause the amount of power transferred to decrease. Or, the MPPT algorithm might repeatedly increase the amount of current Iin, which might cause the amount of power transferred to repeatedly increase, until at some point the amount of power transferred starts to decrease instead of increasing. This would mean that the point of a maximum efficiency power transfer, i.e., the maximum power point, under the present circumstances has been determined. The MPPT algorithm typically would operate so as to maintain the optimum efficiency balance between the input current Iin delivered to the input of the DC-DC converter and the resulting input voltage Vin of the DC-DC converter.
Unfortunately, such prior complex digital MPPT power optimization algorithms consume too much energy to be applicable in nano-power harvesting applications. Various patents, including U.S. Pat. No. 7,564,013 entitled “Method for Matching the Power of a Photovoltaic System to a Working Point at Which the System Produces Maximum Power” issued on Jul. 21, 2009 to Leonhardt et al. and U.S. Pat. No. 7,394,237 entitled “Maximum Power Point Tracking Method and Tracking Device Thereof for a Solar Power System” issued Jul. 1, 2008 to Chou et al., disclose known MPPT power optimization techniques.
Thus, there is an unmet need for a simple, economical technique to achieve maximum power point tracking of a nano-power energy harvester.
There also is an unmet need for an improved MPPT process which is suitable for use in nano-power energy harvesting applications.
There also is an unmet need for an improved circuit and method for avoiding power loss due to mismatch between the output impedance of an energy harvester and the input impedance of a DC-DC converter connected to the energy harvester.
There also is an unmet need for an improved MPPT (maximum power point tracking) process for use in nano-power energy harvesters which itself consumes much less power than prior MPPT processes.
There also is an unmet need for an improved circuit and method for avoiding power loss due to mismatching of energy harvester output impedance to the input impedance of a DC-DC converter over a range of operating conditions.
There also is an unmet need to provide a technique for cost-effective harvesting of energy under conditions wherein the energy required using conventional MPPT techniques exceeds the amount of available energy generated by the harvesting.
There also is an unmet need to provide a technique for controlling the effective input impedance of a DC-DC converter so as to allow it to receive a maximum amount of power from an energy harvester.